The present invention relates to detecting buried. inclusions, and, in particular, land mines with plastic casings. In its broader aspects it is relevant to detecting inclusions buried in granular media whose grains can be described by their elastic constants.
There are millions of unexploded and abandoned land mines. When abandoned land mines are triggered by unsuspecting people, equipment or livestock, they cause unexpected damage. People are often killed or permanently injured by such explosions.
Land mines are used in military conflicts to defend a mined field against an enemy. The mines are often quickly placed. Either poor or no records are made of the placement of the mines. After the conflict is over, the mines remain in place and are a deadly hazard to people, equipment and livestock. Mines that have a metal casing are readily detectable with customary mine sweepers. However, military technology now produces less expensive mines that have plastic or non-metallic casings. Mine sweepers cannot detect such mines. This has created an urgent need for a method and apparatus to detect plastic mines. Such a detection system would save many civilians from death and injury and also protect equipment and livestock.
In a more general sense, it is often desirable to detect buried objects in a granular medium. One often uses electromagnetic detection technologies to locate buried objects in optically opaque media. The success of detection technologies that are based upon electromagnetic waves is contingent upon the presence of materials in the buried objects that allow for the electromagnetic waves to pass through. Non-metallic objects are typically difficult to detect satisfactorily using electromagnetic waves.
An example of a non-metallic buried object is a land mine in a plastic casing, which we will refer to as a xe2x80x9cplastic land mine.xe2x80x9d Plastic land mines contain a minimal amount of metal. The smallest amount of metal in a plastic land mine makes it detectable by conventional means based upon electromagnetic waves. Land mines are designed to avoid detection. Land mine manufacturers have therefore attempted to minimize and even eliminate metal in plastic land mines. The presently available land mines in plastic casings typically contain a very small amount of metal. Such small metal content makes them difficult to detect by conventional electromagnetic means because it is difficult to separate the backscattered electromagnetic signals received from a plastic land mine, from metal scrap and clutter that are inevitably present in the ground.
Arnaud, et al., U.S. Pat. No. 5,808,969 describes a process and device for detecting objects such as land mines using a plurality of acoustic transducers, working in the frequency range 10 Hz to 100 kHz. However, the process and device described in Arnaud were for continuous wave frequencies only, not for impulses, and the patent makes no mention of non-linear acoustics, solitons, or MEMS sensors.
Neff, et al., U.S. Pat. No. 5,412,988 describes an acceleration sensor with a microelectromechanical bender bar used in conjunction with a ferromagnetic core and a superconducting quantum interference device (SQUID). The primary detector in this scheme is the SQUID, which is stated to possess high sensitivity in a preferred frequency range of 1,000 to 10,000 Hz. The present invention does not require a ferromagnetic core or a SQUID sensor.
Other prior art of interest includes U.S. Pat. No. 5,563,848, A. J. Rogers and C. G. Don, titled Object Detector for Detecting Buried Objects and U.S. Pat. No. 5,357,063, L. J. House and D. B. Pape, titled Method and Apparatus for Acoustic Energy Identification of Objects Buried in Soil.
The present invention solves the problem of detecting land mines with little or no metal. The present invention is capable of detecting buried objects or inclusions and of remotely imaging them whether or not there is metal in any of these objects. The detection process in the present invention is based upon newly discovered knowledge concerning (i) the soliton-like propagation of non-linear acoustic impulses in which the acoustic energy being sent through the granular medium suffers very little dispersion or spreading, and (ii) low-frequency continuous acoustic signals. The detection process in the present invention may be applied in granular media consisting of macroscopic elastic grains having a buried inclusion or inclusions. As a result, the invention provides a method and apparatus for locating buried inclusions by sensing the densities that differ from that of the medium itself. The invention also provides a probe that can reveal an image of what is buried in a granular bed without being sensitive to the metal content of the buried object.
The invention uses the following force law that describes the repulsion between two macroscopic elastic grains in contact:
F=naxcex4nxe2x88x921,
where n greater than 2, as shown by H. Hertz in 1881 (Ref. H. Hertz, J. reine u. angew. Math. Vol. 92, p.156 (1881)), and xe2x80x9caxe2x80x9d is a constant that depends upon the Young""s modulus and the Poisson ratio of the materials of the grains in contact.
The present invention uses a soliton-like pulse. That pulse is a non-linear acoustic impulse. It is characterized by some pressure change of desired amplitude (typically less than 0.01 atmospheres) imparted to a surface of a granular bed across a time window of the order of a few micro-seconds or so. We have discovered that at some depth, z (z greater than 10 d), where xe2x80x9cdxe2x80x9d is the average diameter of grains in the medium, from the surface of the three dimensional bed of granular beads in which the pulse has been initiated and in which the adjacent grains repel each other, the pulse can be. approximately described by the following functional form,
"PHgr"n(z)≈A(Y,"sgr")exp[xe2x88x92g(w,xcfx81,z)]tan h(fn(z)/2),
where A(Y, "sgr") is some amplitude of the non-linear acoustic pulse which depends upon the details of the generation of the acoustic pulse and upon the elastic constants characterizing the grains, and g(w, xcfx81, z) is some simple (usually linear) function of xe2x80x9cwxe2x80x9d, xcfx81 and z, where xe2x80x9cwxe2x80x9d denotes the restitutional loss between the granular contacts as the grains load and unload during the propagation of a non-linear acoustic impulse and xcfx81 denotes randomness in the size variation of the grains. The quantity z corresponds to the depth reached by the signal as measured from the surface of the bed.
Typically, these highly non-linear acoustic pulses travel with little dispersion. If one neglects this dispersion, then one may write, z≈ct, where xe2x80x9ccxe2x80x9d is the velocity of the non-linear acoustic pulse and hence relates the depth z to the time of travel t. The argument of the hyperbolic tangent function in "PHgr"n(z) above is,
fn(z)=xcexa3q=0∞C2q+1(n)z2q+1,
where the coefficients C2q+1 (n) depend upon the magnitude of xe2x80x9cn,xe2x80x9d which in turn relates to the geometric and material properties of the grains. See S. Sen and M. Manciu, Discrete Hertzian Chains And Solitons, Physica A, Vol. 268, pp 644-649, 1999. For our purposes, the quantity "PHgr"n(z) describes the relative displacement suffered by any grain, as a non-linear acoustic impulse passes through the grain. The pulse typically causes grain compressions in excess of one-millionth of the diameter of an average grain.